Molds for casting molten metals comprise several mold members working together to define the internal and external shape of the casting. Such members include core members for forming and shaping interior cavities in the casting, as well as cope/drag and shell members for forming and shaping the exterior of the casting. Such mold members are typically made by (1) mixing sand with a binder, (2) introducing (e.g., blowing) the binder-sand mix into a mold containing a pattern (hereinafter "pattern mold") for shaping the sand-binder mix to the desired shape for making the metal casting, and (3) curing/hardening the binder in the pattern mold to harden the binder, and to fix the shape of the mold-forming material (i.e., sand-binder). A variety of synthetic resins are commonly used as binders in so-called "hot-box" and "cold-box" techniques for making such mold members, as is well known to those skilled in the foundry art.
Gelatin has been proposed as a binder for the sand. Heretofore, gelatins, without regard to Bloom ratings, have been used alone or in up to about a 50--50 admixture with certain crystallizeable carbohydrates (e.g., sugar), and baked to form binders for foundry sand--see Solzberg U.S. Pat. No. 2,145,317. Gelatin is desirable because it is water soluble, environmentally benign, and less costly than synthetic resins used in many sand-binder systems. Moreover, less heat is required to break the bonds of the gelatin's protein structure to thermally degrade the binder than is required for the synthetic resin binders. As a result, in the case of mold members which are cores, the gelatin binders break down readily from the heat of the molten metal, and thereby permit ready removal of the core sand from the casting with a minimum of additional processing (e.g., by shaking or hammering. Moreover, because the gelatin is water soluble, any sand that is not removed from the casting mechanically, can readily be washed therefrom with hot water. Solubility of the gelatin also permits ready washing of the binder from the sand for recycling and reuse of the sand to make other mold members and thereby eliminate the cost of new sand.
Gelatin is a proteinaceous material obtained by the partial hydrolysis of collagen, the chief protein component of skin, bone, hides and white connective tissue of animals and is essentially a heterogeneous mixture of polypeptides comprising amino acids including primarily glycine, proline, hydroxyproline, alanine, and glutamic acid. Smaller amounts of other amino acids are also present. Gelatin is sold commercially as a by-product of the meat producing industry. So-called "dry" commercial gelatin actually has about 9% to about 12% by weight water entrained therein, and is an essentially tasteless, odorless, brittle solid having a specific gravity between about 1.3 and 1.4. Gelatins have a wide range of molecular weights varying from about 15,000 to above 250,000, but can be separated one from the other by suitable fractionation techniques known to those skilled in the art. Gelatins are classified, or grouped, into different categories known as "Bloom" ratings or "Bloom" numbers. The Bloom rating or number is determined by the Bloom test which is a system for rating the strength of gels formed from different gelatins. Gelatins having high Bloom ratings/numbers comprise primarily polypeptides with higher average molecular weights than gelatins having lower Bloom ratings/numbers. The Bloom rating/number is determined by evaluating the strength of a gel formed from the gelatin. More specifically, a water solution consisting of 6.67% gelatin is prepared in a specified 150 ml, wide-mouth, glass bottle, which is chilled and held at 10+/-0.1.degree. C. for 17+/-1 hours before testing. After chilling, the rigidity of the gel is measured as the force, in grams, required to impress a standard 0.500+/-0.001 inch diameter plunger to a depth of 4 millimeters into the surface of the gel. This weight in grams is referred to as the Bloom rating or Bloom number of the particular gelatin tested. Commercial gelatins have a broad range of Bloom ratings ranging from about 50 Bloom grams to about 300 Bloom grams. Typically, the viscosity of the gelatin is measured at the same time as the Bloom rating by using the same gelatin sample as is used for the Bloom test. The sample is heated to 60.degree. C. and 100 ml thereof placed in a calibrated capillary pipet. The eflux time from the pipet, in seconds, is recorded and later converted to millipoise. Hence, the viscosity of the gelatin is readily correlated to the Bloom rating, which viscosity is directly proportional to the Bloom number. That is to say, as the Bloom number increases so does the viscosity.
The present invention provides (1) a sand-based, mold-making material, (2) a mold member made therefrom, and (3) a process for manufacturing mold members from such material wherein certain low molecular weight gelatins are selected, and the water content thereof controlled during processing to (a) optimize the strength of the finished mold member while minimizing the gelatin content thereof, and (b) to provide a quick (i.e., within a few minutes) process for making a mold member using commercially available core-making equipment designed for use with resin-bonded sand. As used herein, the term "gelatin" refers to proteinaceous material itself even though so-called "dry" gelatin as it is sold commercially includes about 9% to about 12% entrained water.